This study will provide basic information for determining which eyes of patients with diabetic retinopathy can be successfully treated with photocoagulation. Knowing the status of the retinal capillary system will help to determine when treatment can be delivered most effectively. Using the two-point fluorophotometer, the dye dilution technique will be employed on normal subjects and diabetic patients in order to determine (1) mean circulation time, (2) segmental blood flow, and (3) fluorescein diffusion in the optic nerve and fovea centralis. Diabetics with and without retinopathy will be tested before and after insulin injection or other treatment.